Loss-of-function mutations in members of the neuroligin (NL) family of trans-synaptic cell adhesion molecules have been implicated in human autism and mental retardation. Animal models of autism have been severely limited, but these human genetic findings provide a novel path to develop bona fide mouse models of at least a subtype of human autism or mental retardation. NLs are postsynaptic transmembrane proteins that bind presynaptic beta-neurexins to induce formation of excitatory and inhibitory synapses and to control excitatory/inhibitory (E/I) synapse balance in cultured neurons. Alterations in E/I balance have been proposed as important in pathogenesis of autism and mental retardation. The precise role of NL in vivo and in neurobehavioral abnormalities in autism and mental retardation, however, remains to be determined. We will determine the role of neuroligin in vivo using electrophysiologic and behavioral characterization of NL knockout, human disease mutation knockin, and, in follow-up studies, conditional knockout mice. The driving hypothesis is that mice deficient in NL genes, or carrying known disease-linked mutations in NL, will exhibit behavioral differences consistent with those in human autism or mental retardation, and that these behavioral differences will be associated with specific abnormalities in E/I balance or synaptic function in cortical circuits in vivo. The following specific aims will be addressed: 1. To determine whether NL3 disease-linked mutation or deletion of NL3 result in autism and mental retardation-related behavioral abnormalities. 2. To determine whether deletion of NL3 or NL3 disease-linked mutations result in altered excitatory and inhibitory synaptic connectivity and function. 3. To determine whether deletion of NL3 or NL3 disease-linked mutations alter the threshold for inducing NMDA-receptor-dependent synaptic plasticity in the hippocampus.